Not glasspacks by any chance? They're notorious for the innards rusting and letting the fiberglass plug them up at the outlet.
Dude.... my '76 Dart Lite with the 1 BBL /6 would do 95-100 easy on level ground....and 80 mph up a mile long 9% highway grade in 3rd (Christiansburg Mtn in VA). My dad would do 85 up the same grade in his '56 Ford when we were kids with a stock 292. It ain't just your haids.